1. Field of the Invention
The present invention relates to an electronic type endoscope apparatus for use in the NTSC/PAL systems which can readily cope with video systems having different frame periods.
2. Description of the Prior Art
In recent years, endoscopes have been widely employed which enable an object portion, located deep inside a body cavity, to be observed without requiring a discission by inserting an elongate insertable portion in the body cavity. The endoscopes also enable treatment with instruments, if necessary.
Also, electronic type endoscopes (also called electronic endoscopes or electronic scopes) have recently been put into practical use which do not use an image guide formed by an optical fiber bundle (this type of endoscope is called an optical fiber bundle), and in which an image is converted into electric signals by a solid state imaging element such as a charge coupled device. Picture signals which have been transmitted through a signal cable are displayed on a monitor.
Electronic type endoscopes fall into two categories: a color mosaic type in which a mosaic color filter array is disposed in front of the solid state imaging element so as to obtain a color image of an object which has been illuminated by white light, and a field sequential type in which images obtained by a monochrome solid state imaging element by sequentially illuminating an object by a light having various wavelengths in, for example, the red, green, and blue ranges are superimposed on top of one another to obtain a color image.
The aforementioned field sequential color imaging system uses a rotary color filter 1 such as that shown in FIG. 1 if it is of the type which is used in the NTSC or PAL system.
The rotary color filter 1 has a disk-shaped light shielding plate 2. The light shielding plate 2 has three fan-shaped openings for the purpose of providing a light of three colors. The openings are provided with a red light transmitting filter 3R, a green light transmitting filter 3G, and a blue light transmitting filter 3B, respectively. A portion adjacent to filter 3I (I represents R, G or B) forms a light shielding portion which provides a read-out of signals from an imaging element or a transfer of signals therein. the ratio of the opening for the filter 3I to the light shielding portion is determined so that a) the width of the light shielding portion is set to a minimum value that is required for read out or transfer of signals and b) the width of the opening is set to a maximum value which ensures the white balance of a light comprising the three colors.
In order to display the picture signals output from the imaging element on the TV monitor, the rotary color filter 1 is rotated at a speed which corresponds to a frame repetition rate of 30 frames/second in the NTSC system and at a speed corresponding to a frame repetition rate of 25 frames/second in the PAL system so that the rotation speed is synchronized with TV synchronizing signals, where the same rotary color filter 1 can be used in the both systems. In other words, the rotary color filter 1 is rotated at a speed of 30 turns/second in the NTSC system and 25 turns/second in the PAL system.
In a case where the above-described conventional rotary color filter is employed, the opening time (illuminating time) is longer in the PAL system than that in the NTSC system, which ensures bright illumination. However, color change, which is characteristic of the field sequential method, is more likely to occur in the PAL system than in the NTSC system.
Japanese Patent Laid-Open No. 60-73613 discloses a rotary filter having variable light-shielding plates that can adjust the openings to a suitable value. However, this is not designed to diminish the color change.